1. Field of the Invention
The invention relates to catalysts, and more particularly, to catalysts for the removal of nitrogen oxides from exhaust gases of combustion which are contaminated with dust.
The invention further relates to a catalyst for the separation of the nitrogen oxides from combustion waste gases containing dust by use of ammonia. The catalyst thus exhibits passages for the flow of exhaust gas, and is constructed of individual ceramic moldings, which exhibit a porous surface and a resistance to abrasion such that the dust in the exhaust gas flowing through the passages abrades the catalyst surface, whereupon the dust carries the abraded portions away.
2. Description of the Prior Art
Nitrogen oxides contained in exhaust gases from combustion equipment can be converted, by catalytic reduction using ammonia, into harmless nitrogen and water. For this purpose, the exhaust gases are customarily conducted over bulk catalysts with the simultaneous introduction of gaseous ammonia, liquid ammonia or ammonia dissolved in water.
If the exhaust gases contain large quantities of dust or soot, then the bulk catalyst used quickly becomes stopped up. If the exhaust gases also contain catalyst poisons, such as sulfur or chlorine compounds, then these substances deactivate the cata- lytically-active surfaces. This is especially true if catalysts are used which are sensitive to catalyst poisons
Active catalyst substances and carriers which withstand the various catalyst poisons are expensive, and furthermore, such substances are difficult to dispose of because of their heavy metal content. Catalyst material which is easily disposed of, for example, in a blast furnace, that is, iron oxides, such as Fe.sub.2 O.sub.3, have tendencies toward the above-mentioned catalyst poisoning.
Overall, therefore, dust and catalyst poisons cause the active surface regions of the catalyst to become blocked up quickly, so that the activity of the catalyst is reduced, although there is still active catalyst material below the deactivated surface layer of the catalyst.
Some examples of catalytic technology are found in U.S. Pat. No. 4,259,312, entitled "Process And Apparatus For Catalytically Reacting A Reducing Gas And Water Vapor"; U.S. Pat. No. 4,235,604, entitled "Method For Processing Coke Oven Gas"; and U.S. Pat. No. 4,049,777, entitled "Method Of Waste Gas Treatment", all of which are assigned the same assignee as the instant application.
Further examples of catalytic technology are found in U.S. Pat. No. 4,448,895, entitled "Process For Preparation Of Catalyst For Cleaning Exhaust Gases And Catalyst Prepared For The Process", and U.S. Pat. No. 4,503,162, entitled "Catalyst Systems For The Conversion Of Gases".
Examples of reactivation of catalysts useful in nitrogen oxide removal are described in U.S. Pat. No. 4,044,102, entitled "Method For Treating Exhaust Gases", and U.S. Pat. No. 4,043,939, entitled "Reactivation Of Catalysts Useful For Nitrogen Oxide Removal." All of the above-cited patents are incorporated herein by reference as if the texts thereof were fully set forth herein.